HIV vaccine boost

Scientists have created a vaccine that protects rhesus monkeys from infection by the simian immunodeficiency virus (SIV), a relative of HIV, the virus that causes AIDS. The results will help scientists develop a human version of the vaccine, which could one day halt the global spread of the disease.

Monkeys were used because of the similarities between SIV and HIV. Vaccines work by exposing the body to parts of a virus so that it can learn to recognise them and mount an immune response in the future. Genetically engineered viruses that contain different SIV genes allow researchers to study which part of the virus is most important in provoking an immune response.

The scientists treated groups of rhesus monkeys with several different 'prime-boost' vaccine combinations. The 'prime' stage was a virus vector genetically engineered to include some DNA from SIV genes. The second stage, the 'boost', given about six months later, comprised another vector expressing the same genes. Six months after the boost, the researchers repeatedly infected the monkeys with a strain of SIV that was different to the one in the vaccine, and against which they knew the monkeys' immune system would not be able to respond strongly on its own.

The combination that worked best at preventing infection was one where the prime vector was an adenovirus and the boost was a modified-pox-virus. Three quarters of non-vaccinated monkeys developed SIV, compared with only one in eight that had received the best prime-boost vaccine.

The researchers analysed the antibodies produced in response to the vaccine. Monkeys that were resistant to SIV infection had high levels of antibodies against the virus ENV coat proteins, as do HIV-resistant people, confirming that boosting these proteins is essential to any successful vaccine programme. Clinical trials to test similar HIV vaccine combinations in healthy human adults are already at the planning stage.